A gas barrier film, in which a plurality of layers including metal oxide thin films such as an aluminum oxide thin film, a magnesium oxide thin film, a silicon oxide thin film and so forth are laminated and formed on the surface of a plastic substrate or film has been widely utilized conventionally for packaging application to pack goods necessary for shielding various gases such as water vapor, oxygen and so forth, and to avoid alteration in quality of food, industrial goods, pharmaceutical products and so forth.
Further, in those other than packaging application, it has been used as a solar cell element, an organic electroluminescence (EL) element, a liquid crystal display element or the like.
As methods each of forming such a gas barrier film, known are a chemical vapor deposition method (a plasma CVD method: Chemical Vapor Deposition) by which a film is formed on a substrate during oxidation with oxygen plasma at reduced pressure, employing an organic silicon compound typified by tetraethoxy silane (TEOS), and a sputtering method by which metal silicon is evaporated employing a semiconductor laser to deposit it on a substrate in the presence of oxygen.
These methods have been desirably used for formation of metal oxide thin films such as a SiO2 thin film and so forth, since a thin film having an accurate composition can be formed on a substrate, but poor productivity has largely resulted since time is consumed on the ground of reduced pressure and opening to the atmosphere, difficult continuous production, and large-size equipment to be used because of deposition at reduced pressure.
In order to solve these problems, utilized are a method by which a silicon-containing compound is coated to form a silicon oxide thin film via modification of the coating film, and the same CVD method as previously mentioned in which plasma is generated under atmospheric pressure to form a film, and a gas barrier film has also been applied thereto, and studied.
As a silicon oxide film which can be generally prepared by a solution process, known is a preparation technique called a sol-gel method employing an alkoxide compound as raw material. In this sol-gel method, heating to high temperature is generally necessary, and large contraction in volume further occurs in the process of dehydration-condensation reaction, resulting in generation of a large number of defects in the film.
In order to avoid the foregoing, a method by which an organic substance or the like directly having no involvement in formation of oxide is mixed in a raw material solution is known, but when regarding a film as a whole, it lowers a barrier property that each of these organic substance remains in the film, resulting in appearance of an insufficient barrier property.
For this reason, it was difficult to use an oxide film to be prepared by a sol-gel method as a protective film for a flexible electronic device as it is.
As another method, it is proposed that a silicon oxide film is formed using a silazane compound having a silazane structure (Si—N) as a basic structure in raw material.
Since reaction in this case is not dehydration-condensation reaction but direct substitution reaction from nitrogen to oxygen, mass yield before and after the reaction is a large value such as 80% to 100%, whereby it is known that a dense film having reduced defects in the film, caused by contraction in volume.
However, since temperature higher than 450° C. should be applied for formation of a silicon oxide film via substitution reaction of the silazane compound, it is impossible that a flexible substrate made of plastic or the like is suitably applicable.
As a means to solve such a problem, proposed is a method by which a coating film having coated and formed from a silazane compound is exposed to vacuum UV radiation to form a silicon oxide film.
Oxidation reaction with active oxygen or ozone can be accelerated while directly cutting via action with only photon called a photon process for atomic binding, employing light energy having a wavelength of 100-200 nm called vacuum UV radiation (hereinafter, referred to also as “VUV” or “VUV radiation”) larger than interatomic binding force inside the silazane compound to form a silicon oxide film at considerably low temperature.
From the viewpoint of preparation of a gas barrier film, the gas barrier film is industrially desired to be continuously manufactured by a so-called roll-to-roll system.
Thus, as a manufacturing method employing the roll-to-roll system, known is a method by which a film is conveyed at a speed of 1 min/min or 10 m/min, and a silazane compound-coating film is exposed to an excimer lamp to prepare a gas barrier film (refer to Patent Document 1 and Nonpatent Document 1).
However, in these methods, there appeared a problem such that productivity was insufficient, and barrier performance of a gas barrier film was often insufficient.